Small-signal modeling of MMC based DC grid and analysis of the impact of DC reactors on the small-signal stability

This paper proposed a generic modeling framework of the modular multilevel converter (MMC) based DC grid for small-signal stability studies. A modularized modeling method is employed to form the overall state-space model and small-signal model with considering the detailed dynamics of MMC internal electrical quantities, MMC control patterns and the DC network. The validity and accuracy of the state-space model and small-signal model for a five-terminal MMC system are verified versus the electromagnetic transient model in PSCAD/EMTDC. Based on the small-signal model, the impact of the DC reactors on the small-signal stability of the tested system is investigated through root locus analysis. Key DC reactors that have a dominant effect on the stability are identified. Critical values of the DC reactors that preserve stability are recognized. Damping control is designed and installed according to the participation factor analysis. The time-domain simulation results show excellent agreement on the oscillating frequency calculated by eigenvalue studies and verify the validity of the proposed damping control.